1. Field of the Invention
The present invention relates to a camera having a two lens group zoom system composed of a front lens group and a rear lens group, and more particularly to home position check of the rear lens group driven by a stepping motor.
2. Related Background Art
A recent demand is to reduce the outer dimensions of camera, particularly of compact camera. Especially, an important subject is to miniaturize a lens barrel as much as possible. To miniaturize the lens barrel, a recent trend in camera having the two lens group zoom system is to replace a mechanical cam system with a mechanism for driving the front lens group and the rear lens group by separate motors. In such a mechanism, it is general that a direct current motor is used as a motor for driving the front lens group and a stepping motor as a motor for driving the rear lens group.
An example of the two lens group zoom system is one as shown in FIGS. 1A and 1B. A lens barrel 4 of the zoom system 2 is composed of a stationary barrel 6 forming a part of camera body 7, an intermediate barrel 8 telescopically stored in the stationary barrel 6, and a movable barrel 10 telescopically stored in the intermediate barrel 8.
A front lens group (FLG) 22 which consists of one or more lenses is fixed in a front end portion of the moving barrel 10 and is arranged to be driven by a direct current (DC) motor (not shown in FIGS. 1A and 1B) to move backward and forward together with the moving barrel 10.
Also, a rear lens group (RLG) 28 which consists of one or more lenses is arranged behind the front lens group 22 (on the camera body side) in the moving barrel 10 so as to be movable backward and forward. The rear lens group 28 is driven backward and forward by a stepping motor (not shown in FIGS. 1A and 1B).
In such a zoom system 2, a home position (reference position) is defined at a position where the rear lens group 28 is located closest to the front lens group 22 and the position of rear lens group 28 is obtained from a cumulative number of pulse signals (hereinafter referred to as "rear group cumulative pulse number") output to the stepping motor to move the rear lens group 28 from the home position. Whether the rear lens group 28 is located at the home position is detected using a home position sensor composed of a limit switch, a photoelectric sensor or the like.
The conventional zoom systems as described above could have a chance that an actual position of rear lens group is different from a position thereof obtained from the rear group cumulative pulse number if there occurs an abnormality in drive of stepping motor for example because of a decrease in drive voltage. If this positional deviation, i.e., step-out, occurs, the rear lens group cannot be accurately located at the focusing position during exposure, which would cause a problem of out-of-focus, for example.
Therefore, the conventional systems are so arranged that for example when the lens barrel is moved from the collapsed or retracted position to a photographable range or when it is retracted from the photographable range to the retracted position, the rear lens group is moved to the home position and home position check is executed to detect whether the rear lens group is properly located at the home position (that is, it is detected whether the home position sensor is on or off).
However, in the home position check in the conventional systems, the on/off detection of home position sensor is executed every predetermined pulse number movement (for example every unit of four pulses) of rear lens group, which results in performing unnecessary processes while the rear lens group is located considerably apart from the home position. Therefore, an excessive time could be needed for a shift from the home position check process to a next process.
Consequently, a need exists for a way to perform the home position check within as short time as possible.